1. Field of the Invention
The present invention relates to a ceramic sintered body, more specifically to an aluminum nitride sintered body, various members using this sintered body, and a method of manufacturing the aluminum nitride sintered body.
2. Description of the Related Art
Aluminum nitride sintered bodies have favorable heat resistance and corrosion resistance as well as high thermal conductivity, and have been heretofore used widely as a base substance material for an electrostatic chuck for fixing a wafer, a ceramic heater for heating the wafer, and the like in semiconductor-manufacturing apparatuses such as a plasma etching apparatus or a plasma chemical vapor deposition (CVD) apparatus.
In terms of an application as the electrostatic chuck, Johnsen-Rahbek force is mainly used as chucking force at the moment. To obtain favorable chucking force, this electrostatic chuck requires a base substance material having relatively low volume resistivity in a range from 108 to 1012 Ωcm. However, aluminum nitride per se is a high-resistance material having volume resistivity at room temperature equal to or above 1014 Ωcm, and is therefore subject to reduction in a value of resistance.
The applicant has heretofore developed an aluminum nitride sintered body having reduced resistance in a range from about 108 to 1012 Ωcm by means of adding a rare-earth oxide such as yttrium oxide, cerium oxide or samarium oxide to an aluminum nitride sintered body.
As described above, in the base substance material used for the electrostatic chuck, it is necessary to obtain the volume resistivity in a range from about 108 to 1012 Ωcm to obtain the chucking force based on Johnsen-Rahbek's principle. However, among ceramic members used in semiconductor-manufacturing apparatuses, there is a case where a lower value of resistance is required depending on an application.
For example, in a plasma etching apparatus or the like, a ring-shaped ceramic member is placed around the electrostatic chuck to prevent the base substance of the electrostatic chuck from corrosion by halide gas. An insulative ceramic has been conventionally used for this ring-shaped member. However, to generate uniform and stable plasma on a wafer to be placed on the electrostatic chuck, it is desired to use a material having the volume resistivity equivalent to that of the wafer as the ring-shaped member which is exposed around the wafer. Accordingly, it is necessary to provide the base substance material for the ring-shaped member with electric conductivity equal to or below 104 Ωcm, which represents a semiconductor region equivalent to a silicon wafer, for example. Moreover, a ceramic member having higher electric conductivity can diversify usability not only for semiconductors but also for various applications as an electrically conductive member provided with corrosion resistance, heat resistance and strength.
For example, a method of adding an electrically conductive material such as titanium nitride (TiN) with an insulative ceramic material is known as a method of reducing a value of resistance of a ceramic. However, to obtain the volume resistivity equal to or below 104 Ωcm by this method, a large amount of the electrically conductive material equal to or above 20% must be added because it is necessary to form electrically conductive paths inside the ceramic material by use of the electrically conductive material.
However, when such a large amount of the electrically conductive material is added, it is difficult to maintain properties of the ceramic material being a mother material. For example, when aluminum nitride is used as the mother material, there is a risk of damaging the high thermal conductivity, the heat resistance, and halogen resistance of aluminum nitride. Therefore, to maintain the properties of the mother material, it is desired to use an additive material which can reduce the value of volume resistivity by adding the material as little as possible.
In addition, use of a material having even lower resistance equal to or below 10 Ωcm is expected depending on certain use conditions. In this case as well, the material is expected to maintain the corrosion resistance, the heat resistance and the strength required for an application. Accordingly, it is preferable that an amount of the additive material remains within a range which can maintain these properties.